Open Messages or Photos on your iPhone
Tap and hold the subject in a photo
Release when the outline appears
Tap Add Sticker
Open a chat in Messages
Tap the App Store icon next to the text field
Tap the sticker icon
Select your sticker
Drag and drop it into the conversation
To make a custom sticker, open a photo
Tap and hold the subject
Tap Add Sticker
Tap the sticker in Messages to edit or reuse it
Use Markup in Photos to draw and save a sticker-like image
Use a third-party sticker app from the App Store if needed